1. Field of the Invention (Technical Field)
The present invention relates to photovoltaic solar cells for the generation of electrical power directly from light, whether natural sunlight or artificial light, and more particularly, to thin crystalline substrate solar cells employing emitter wrap-through (EWT) means wherein a conductive channel is formed through the silicon wafer in order to electrically contact an emitter on the front surface by wrapping the emitter through the thin crystalline substrate.
2. Background Art
Photovoltaic solar cells in use today typically are based on either crystalline silicon technology or on one of a variety of thin film technologies, such as amorphous silicon, copper indium diselenide, or cadmium telluride. Crystalline silicon has certain advantages over the thin films. The primary advantages of crystalline silicon include energy conversion efficiency, which is higher, and the durability and reliability when used out of doors. Thin films suffer from lower energy conversion efficiency, especially when fabricated on a commercially viable scale, and from degradation in performance when used out of doors for an extended period of time. Due to these fundamental problems, crystalline silicon is used in over 85% of the outdoor applications.
The current state-of-the-art in the photovoltaics industry is a solar cell fabricated on a thick, greater than 300 microns, crystalline silicon wafer. The wafer can be single crystal or multicrystalline. The solar cell design in widespread use today has a p/n junction formed near the front surface (that surface which receives the light) which creates an electron flow as light energy is absorbed in the cell. The conventional cell design has one set of electrical contacts on the front side of the cell, and a second set of electrical contacts on the back side of the solar cell. In a typical photovoltaic module these individual solar cells are interconnected electrically in series to increase the voltage. This interconnection is typically accomplished by soldering a conductive ribbon from the front side of one solar cell to the back side of an adjacent solar cell.
The current invention makes use of a different cell design called an Emitter Wrap-Through EWT) solar cell. The EWT cell is one approach in a family of designs called back-contact cells, all of which have both sets of electrical contacts on the back of the cell. These approaches are well documented, and include not only EWT but also Metal Wrap Through (MWT), Metal Wrap Around (MWA), and back junction designs. The unique feature of EWT cells, in comparison to MWT and MWA cells, is that there is no metal coverage on the front side of the cell, which means that none of the light impinging on the cell is blocked. The unique feature of the EWT cell in comparison to back junction solar cells is that an EWT cell maintains a current collection junction on the front surface, which is advantageous for high current collection efficiency. These advantages, in turn, lead to increased electrical output. The EWT cell is disclosed in U.S. Pat. No. 5,468,652, Method Of Making A Back Contacted Solar Cell, to James M. Gee, incorporated here in full. The various back contact cell designs have also been discussed in numerous technical publications. However, all previous MWT, MWA, and EWT back contact cell designs have employed silicon wafers of standard thickness, above about 300 microns, while back junction cells require the use of expensive silicon materials with exceptionally long lifetimes.
In addition to U.S. Pat. No. 5,468,652, two other U.S. patents on which Gee is a co-inventor disclose methods of module assembly and lamination using back-contact solar cells, U.S. Pat. No. 5,951,786, Laminated Photovoltaic Modules Using Back-Contact Solar Cells, and U.S. Pat. No. 5,972,732, Method of Monolithic Module Assembly. Both patents disclose methods and aspects that may be employed with the invention disclosed herein, and are incorporated by reference as if set forth in full. U.S. Pat. No. 6,384,316, Solar Cell and Process of Manufacturing the Same, discloses an alternative back-contact cell design, but employing MWT, wherein the holes or vias are spaced comparatively far apart, with metal contacts on the front surface to help conduct current to the back surface, and further in which the holes are lined with metal.
Conventional crystalline silicon solar cells with contacts on both the front and back surfaces have disadvantages. A thick silicon wafer is required in order to provide the necessary strength for the manufacturing processes and resultant stresses. As wafers are made thinner, they are unable to accommodate the strain due to the coefficient of thermal expansion mismatch between the wafer and the back surface field (BSF), typically comprising an aluminum alloy on the back surface of the wafer. The purpose of the BSF is to reduce recombination losses (“passivation”) at the back surface of standard configuration solar cells. The Al must be thick (typically greater than 30 μm) and have full-area coverage in order to achieve the desired electrical performance. However, the thermal expansion coefficient of Al is over 10× larger than that of Si. The resultant stress causes bowing of the cell, which can rise exponentially as the wafer thickness decreases, dramatically reduces manufacturing yields. Alternative passivation techniques to using a thick Al layer, such as reducing the Al thickness or firing temperature, using a thin-film evaporated metallization, using various dielectric layers (for example, thermally grown silicon dioxide or deposited layers of silicon dioxide, silicon nitride, etc.), using semiconductor heterojunctions (such as amorphous silicon or polysilicon), or using a boron doped silicon layer rather than the Al alloyed BSF, have not equaled the passivation of the Al layer and/or are expensive and difficult to perform, thus detracting from the cost savings obtained by using a thin wafer. These disadvantages are disclosed in, for example, A. Schneider et al., “Al BSF For Thin Screenprinted Multicrystalline Si Solar Cells”, presented at the 17th Eur. PV Solar Energy Conf., Munich, October 2001; A. Schneider et al., “Bow Reducing Factors For Thin Screenprinted Mc-Si Solar Cells With Al BSF”, presented at the 29th IEEE Photovoltaic Specialists Conference, New Orleans, La., May 2003 (p. 336), and F. Duerinckx et al., “Improved Screen Printing Process For Very Thin Multicrystalline Silicon Solar Cells”, Presented at the 19th EPVSEC, 2004, Paris.
Typically the thickness of silicon wafer solar cells, whether back-contact or not, is over 300 microns. The amount of silicon required is a significant proportion of the cost of a conventional solar cell, and is a barrier to dramatic cost reduction required for more widespread use of photovoltaic power generation. Although thin films have the theoretical advantage of reducing the amount of raw material required, as the thickness of the semi-conducting layers is typically on the order of 1-5 microns, they have not been able to overcome the problems of low efficiency, poor reliability and environmental degradation. An alternative cell structure described in U.S. Pat. No. 6,143,976, Solar Cell with Reduced Shading and Method of Producing the Same, comprises a “tricrystal” wafer design, which because of the particular design and orientation of internal crystal angles, may be sawed comparatively thin. However, the disclosure is limited to the particularly tricrystal wafer design, and requires specialized etching methods and protocols. By requiring a specific crystalline orientation, the grid structure that must be used is convoluted and requires significant passivation of the back surface. It is further disclosed that a back surface recombination velocity of <100 cm/s, is required, which is very difficult if not impossible to achieve.
There is thus a need in the industry for solar cell designs which employ a thin crystalline wafer, such that the wafer is less than 300 microns in thickness, and preferably significantly less than 300 microns in thickness.